In recent years, in the field of semiconductor device, especially in the field of display device, there are demands for further improvement in performance without an increase in cost. Specifically, there are demands for formation of higher-performance elements (e.g., semiconductor elements formed from a monocrystalline semiconductor) on a large-sized substrate of a semiconductor device or a display device at lower cost.
In such fields, there have been attempts to use, as a device substrate, a film substrate or the like, which has high impact resistance, light weight, and flexibility, in order to allow the device to have improved impact resistance, reduced weight, and flexibility.
However, such a film substrate has a low heat resistance of approximately 200° C. Currently, a process using amorphous silicon or polycrystalline silicon which involves a high temperature (approximately in a range from 350° C. to 600° C.) process is widely used as a semiconductor element formation process. It is difficult to directly apply this semiconductor element formation process to formation of semiconductor elements on a film substrate.
Although it is of course possible to form semiconductor elements on a film substrate by using a low temperature (200° C. or less) process, properties of semiconductor elements formed by such a low temperature process are remarkably inferior to those of semiconductor elements formed by a high temperature process.
On this account, there are demands that semiconductor elements formed by using a high temperature process be provided on a film substrate.
A method using a transfer process is known as a method for allowing high performance elements to be provided on a large-sized substrate of a semiconductor device or a display device at low cost, e.g., a method for allowing semiconductor elements formed by using a high temperature process to be provided on a film substrate which has low heat resistance of approximately 200° C.
It is difficult to form, for example, a monocrystalline semiconductor layer, which is necessary for formation of high-performance elements, directly on a large-sized substrate of a semiconductor device or a display device. However, use of the transfer process eliminates the need for such direct formation which causes an increase in cost.
Specifically, a substrate having a monocrystalline semiconductor layer of a size that is relatively easy to form is prepared. Then, semiconductor elements are formed from the monocrystalline semiconductor layer in high density. Subsequently, the semiconductor elements thus formed in high density are transferred onto a large-sized substrate of a semiconductor device or a display device. In this way, the semiconductor elements formed from the monocrystalline semiconductor can be provided on the large-sized substrate of the semiconductor device or the display device at low cost.
Moreover, use of the transfer process allows semiconductor elements to be transferred onto a low-heat-resistance film substrate after the semiconductor elements are formed on a high-heat-resistance substrate by using a high temperature process. In this way, the semiconductor elements formed by using the high temperature process can be provided on the low-heat-resistance film substrate.
For example, Patent Literature 1 discloses a method for producing a liquid crystal display device with the use of a transfer process.
FIG. 7 is a diagram explaining a process of forming thin film transistors 102 on a substrate 301 with the use of a transfer process.
Explained first is a transfer process of transferring thin film transistors 102 from an element formation substrate 401, which is a substrate on which the thin film transistors 102 are first formed, onto an intermediate transfer substrate 701. Explained next is a transfer process of transferring the thin film transistors 102 from the intermediate transfer substrate 701 onto the substrate 301.
First, as illustrated in FIG. 7, an etching stopper layer 402 and an undercoat layer 305 are successively laminated on the element formation substrate 401. The thin film transistors 102 are formed on the undercoat layer 305. Further, protection films 601 are formed so as to cover the respective thin film transistors 102.
Then, dry-etch or the like is performed to remove the etching stopper layer 402 and the undercoat layer 305 except for regions covered by the protection films 601 which are formed so as to cover the respective thin film transistors 102. Thus, the etching stopper layer 402 and the undercoat layer 305 are separated into portions corresponding to the respective thin film transistors 102.
Meanwhile, on the intermediate transfer substrate 701, light absorbers 702 are provided on positions corresponding to the respective thin film transistors 102. Further, an adhesive/release layer 703 is provided so as to cover the intermediate transfer substrate 701 and the light absorbers 702.
Then, as illustrated in FIG. 7, after the light absorbers 702 and the protection films 601 are aligned with each other, the adhesive/release layer 703 and the protection films 601 are bonded to each other. Subsequently, the element formation substrate 401 is etched with the use of a mixture solution of hydrofluoric acid and a surfactant in a state in which side surfaces of edge portions of the intermediate transfer substrate 701 are protected by a tape or the like. Note that the etching is adjusted not to go beyond the etching stopper layer 402.
The thin film transistors 102 can be thus transferred from the element formation substrate 401 onto the intermediate transfer substrate 701.
The following describes the transfer process of transferring the thin film transistors 102 from the intermediate transfer substrate 701 onto the substrate 301.
As illustrated in FIG. 7, signal lines 104 and scanning lines 105 are provided on the substrate 301. An interlayer insulating film 302 for insulating the signal lines 104 from the scanning lines 105 is formed so as to cover the signal lines 104. Further, a planarizing film 303 is formed so as to cover the scanning lines 105 and the interlayer insulating film 302.
Note that a contact portion 201 is formed in part of a region above each of the signal lines 104.
Then, an adhesive layer 1501 for bonding a thin film transistor 102 to be transferred is formed on the planarizing film 303.
As illustrated in FIG. 7, the position of the intermediate transfer substrate 701 is adjusted so that the intermediate transfer substrate 701 is located above the adhesive layer 1501, and the thin film transistor 102 to be transferred is bonded to the adhesive layer 1501.
Then, an upper portion of the thin film transistor 102 to be transferred is selectively irradiated with light through the intermediate transfer substrate 701, so as to heat the light absorber 702. The heat reduces adhesion of the adhesive/release layer 703, thereby separating the thin film transistor 102 from the intermediate transfer substrate 701. As a result, the thin film transistor 102 is bonded to the substrate 301 side.
Thereafter, an ITO film is patterned to simultaneously form a connecting electrode 203, which is for connecting the signal line 104 and the thin film transistor 102, and a pixel electrode 103. In this way, an active matrix substrate 101 is produced.
FIG. 8 is a diagram illustrating an outline configuration of a liquid crystal display device including the active matrix substrate 101 produced by the process illustrated in FIG. 7.
As illustrated in FIG. 8, in the thin film transistor 102, the undercoat layer 305, a gate electrode 306, a gate insulating film 307, a semiconductor layer 308, and a channel protection insulating film 309 are laminated in this order. On a portion of the semiconductor layer 308 from which the channel protection insulating film 309 is removed, an n-type semiconductor layer 310 is provided. On the n-type semiconductor layer 310, a source electrode 311 and a drain electrode 312 are provided. On the source electrode 311 and the drain electrode 312, a passivation film 313 is provided. Further, contact holes 314 are provided in portions corresponding to the source electrode 311 and the drain electrode 312. The active matrix substrate 101 obtained by the above process in which the thin film transistor 102 thus arranged is formed on an adhesive layer (not illustrated) is combined with a counter glass substrate 2403 on which a color filter 2401 and a counter electrode 2402 are provided. Further, a liquid crystal layer 2404 is injected between the two substrates 101 and 2403. In this way, the liquid crystal display device can be produced.
As described above, in such a liquid crystal display device including the active matrix substrate 101 produced by the transfer process, high-performance elements can be provided at low cost. Moreover, semiconductor elements formed by using a high temperature process can be provided on a film substrate which has low heat resistance.